In general, the non-linear optical effect means a non-linear optical response proportional to the square, cube or higher power of photoelectric field applied. Known examples of the secondary non-linear optical effect proportional to the square of photoelectric field applied include second harmonic generation (SHG), optical rectification, photorefractive effect, Pockels effect, parametric amplification, parametric oscillation, light sum frequency mixing and light difference frequency mixing. Examples of the ternary non-linear optical effect proportional to the cube of photoelectric filed applied include third harmonic generation (THG), optical Kerr effect, self-induced refractive index change and two-photon absorption.
As the non-linear optical material of exhibiting these non-linear optical effects, a large number of inorganic materials have been heretofore found. However, inorganic materials can be hardly used in practice because so-called molecular design so as to optimize desired non-linear optical characteristics or various properties necessary for the production of a device is difficult. On the other hand, organic compounds can realize not only optimization of desired non-linear optical characteristics by the molecular design but also control of other various properties and therefore, the probability of its practical use is high. Thus, organic compounds are attracting attention as a promising non-linear optical material.
In recent years, among non-linear optical characteristics of the organic compound, ternary non-linear optical effects, particularly, non-resonant two-photon absorption, are being taken notice of. The two-photon absorption is a phenomenon such that a compound is excited by simultaneously absorbing two photons. In the case where the two-photon absorption occurs in the energy region having no (linear) absorption band of the compound, this is called non-resonant two-photon absorption. In the following, even when not particularly specified, two-photon absorption indicates non-resonant two-photon absorption.
The non-resonant two-photon absorption efficiency is proportional to the square of photoelectric field applied (square-law characteristic of two-photon absorption). Therefore, when a laser is irradiated on a two-dimensional plane, two-photon absorption takes place only in the position having a high electric field strength at the center part of laser spot and utterly no two-photon absorption occurs in the portion having a weak electric field strength in the periphery. On the other hand, in a three-dimensional space, two-photon absorption occurs only in the region having a large electric field strength at the focus where the laser rays are converged through a lens, and two-photon absorption does not take place at all in the off-focus region because the electric field strength is weak. As compared with the linear absorption where excitation occurs in all positions proportionally to the strength of photoelectric field applied, in the non-resonant two-photon absorption, excitation occurs only at one point inside the space by virtue of the square-law characteristic and therefore, the space resolution is remarkably enhanced.
Usually, in the case of inducing non-resonant two-photon absorption, a short pulse laser in the near infrared region having a wavelength longer than the wavelength region where the (linear) absorption band of a compound is present, and not having the absorption of the compound is used in many cases. Since a near infrared ray in a so-called transparent region is used, the excitation light can reach the inside of a sample without being absorbed or scattered and one point inside the sample can be excited with very high space resolution due to the square-law characteristic of non-resonant two-photon absorption.
Therefore, if polymerization can be caused by using the excitation energy obtained upon non-resonant two-photon absorption, polymerization can be brought about at an arbitrary position in a three-dimensional space and this enables application to a three-dimensional optical recording medium, a fine three-dimensional stereo-lithography material and the like, which are considered as an ultimate high-density recording medium.
Examples of the technique of performing two-photon photopolymerization by using a non-resonant two-photon absorbing compound and applying it to stereolithography and the like are described in B. H. Cumpston et al., Nature, Vol. 398, page 51 (1999) [Non-Patent Document 1], K. D. Belfield et al., J. Phys. Org. Chem., Vol. 13, page 837 (2000) [Non-Patent Document 2], C. Li et al., Chem. Phys. Lett., Vol. 340, page 444 (2001) [Non-Patent Document 3], K. D. Belfield et al., J. Am. Chem. Soc., Vol. 122, page 1217 (2000) [Non-Patent Document 4], S. Maruo et al., Oppt. Lett., Vol. 22, page 132 (1997) [Non-Patent Document 5].
However, these techniques have the following problems:
1) the two-photon absorbing cross-sectional area of the two-photon absorbing compound is small,
2) two photons are absorbed directly into a polymerization initiator having a very low two-photon absorbing cross-sectional area, without using a two-photon absorbing compound,
3) a polymerization initiator is not used,
4) the polymerization initiator, if used, has bad matching with the two-photon absorbing compound, and the like. In this way, a high-efficiency two-photon absorbing compound and an appropriate polymerization initiator are not used and this gives rise to problems in practice, that is, the polymerization efficiency is bad and for performing stereolithography or the like by polymerization, a strong laser must be irradiated for a long period of time.
[Non-Patent Document 1]
B. H. Cumpston et al., Nature, Vol. 398, page 51 (1999)
[Non-Patent Document 2]
K. D. Belfield et al., J. Phys. Org. Chem., Vol. 13, page 837 (2000)
[Non-Patent Document 3]
C. Li et al., Chem. Phys. Lett., Vol. 340, page 444 (2001)
[Non-Patent Document 4]
K. D. Belfield et al., J. Am. Chem. Soc., Vol. 122, page 1217 (2000)
[Non-Patent Document 5]
S. Maruo et al., Oppt. Lett., Vol. 22, page 132 (1997)
As described above, if polymerization can be caused by using the excitation energy obtained upon non-resonant two-photon absorption, polymerization can be brought about at an arbitrary position in a three-dimensional space with very high space resolution and this enables application to a three-dimensional optical recording medium, a fine three-dimensional stereolithography material and the like, which are considered as an ultimate high-density recording medium. However, two-photon absorbing compounds usable at present are low in the two-photon absorbing ability and polymerization initiating ability and also have bad matching with a polymerization initiator and therefore, the polymerization efficiency is extremely low. As a result, a very high-output laser is necessary as a light source and the recording takes a long time.
In particular, for use in a three-dimensional optical recording medium, it is essential to establish a two-photon absorbing polymerizable composition capable of undergoing photopolymerization with high sensitivity and thereby attain a high transfer rate.